DE 26 13 481 A1 discloses a filter unit for oil for an engine, consisting of a filter insert, which is provided with a pleated paper filter element held between end caps that are spaced a distance apart from one another axially. The filter unit is equipped with an annular baffle extending axially within the insert between the inside of the base plate and a curved end section of a housing in the position between the radial exterior side of a filter element and an interior circumference of the insert. With the interior circumference of the side of the insert, the baffle borders a ring or an annular chamber, which can be connected to the interior of the insert through a plurality of orifices disposed at a circumferential distance and an axial distance. An additional body designed in an annular form is disposed inside the ring, this additional body being manufactured from a suitable polymer product with an additive incorporated into it. The additional body may be made of any one of a plurality of polymer products having a high molecular weight, which will dissolve slowly in oil and into which the desired oil additives can be incorporated, so that on contact of the additional polymer product composition by the oil, the polymer product has a slow dissolving rate in the oil and therefore dissolves slowly and/or is dispersed slowly in the oil. The polymer product thus serves two important purposes: it is the carrier medium for the additives and it also protects the additives from direct contact with the oil. The term “additive” includes all the materials which can be mixed with the polymer product or admixed into it and which impart properties that are beneficial in some regard to the oil circulated through the filter unit.